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The Mac Plus 30 Years On

Daniel Knight - 2016.01.24

Prior to 1986, the best Mac had 512 KB of memory with no expansion path, a 400 KB floppy drive, and no standard way of connecting a fast hard drive. The Mac Plus, introduced on January 16, 1986, changed all that.

There were third-party upgrades for the Mac 512K, also known as the Fat Mac for having four times the RAM of the original Mac. There were memory cards that might add 2 MB, there were some SCSI cards for external devices, and there were internal third-party drives such as the GCC HyperDrive. Apple even had the Hard Disk 20, a 20 MB hard drive that connected to the Mac’s slow (500 Kb/s) floppy drive port.

The Mac Plus shipped with 1 MB of RAM, which could be upgrade to 4 MB using 30-pin SIMMs – so much easier than installing individual chips!

SCSI

But most of all, the Mac Plus included something else brand new to Macs – SCSI. SCSI is a standardized peripheral interface that can support hard drives, removable media drives, scanners, and printers, as well as other devices. The Mac Plus can support up to 7 SCSI devices, making it by far the most expandable Mac at its introduction.

SCSI was brand new in 1986, and its implementation on the Mac Plus didn’t allow it to reach its full potential. Then again, neither did many early hard drives. This was before hard drives included a data buffer, so to keep drives from sending data too quickly for the computer, data sectors were interleaved so there might be one, two, three, or even more sectors between one data block in a file and the next one. For the Mac Plus, two sectors between used sectors was fairly common.

Once hard drives had built-in buffers, there was no more need to interleave when formatting hard drives. The buffer would transfer the data no faster than the computer could receive it. That made a world of difference for Mac Plus hard drive performance.

With the optimal interleave set and Apple’s standard drivers, the Mac Plus can handle drive data at up to 2.1 Mb/s – over four times the maximum bandwidth of the Hard Disk 20.

Other Improvements

The Mac Plus came with updated ROMs that included support for SCSI hard drives and 800 KB floppies, as well as the Mac’s new Hierarchical File System (HFS). The new ROMs are 128 KB; earlier Macs had 64 KB of ROM. They also included support for booting directly from the Hard Disk 20, which only worked on the Fat Mac when pre-booted from a floppy disk.

The Mac Plus was the first Mac to include a numeric keypad and arrow keys on its keyboard.

The Mac Plus was the first Mac to use round serial ports, a style Apple continued to use until USB replaced it starting in 1998.

The Mac Plus is the oldest Mac that can run System 7, which requires at least 2 MB of memory and a hard drive with at least 1.2 MB of free space for a minimum install. System 7 originally shipped on 9 800 KB floppy disks, version 7.1 on 11, and version 7.5 on 13. System 7.1 also needs 1.2 MB of hard drive space for a minimum install, and 7.5.5, the highest version to run on a Mac Plus, requires 5.0 MB.

When the Mac Plus was finally discontinued on October 15, 1990, it set a record that exists to this day for a single Macintosh model. It was on the market for 4 years and 9 months.

Mine has been upgraded to 4 MB of RAM and a 16 MHz 68000 Brainstorm accelerator, and after all these years, it still starts up without a hitch.